1. Field of the Invention
This invention relates generally to the field of measuring instruments and, more particularly, to a process and apparatus for determining the heating value per unit volume of a gaseous fuel, such as natural gas.
2. Background Information
Conventionally, gaseous fuel is sold on the basis of its heating value or net heat content. Measurement of the heating value of a distributed fuel, such as natural gas, is typically accomplished by testing a sample of the gas in a laboratory utilizing complex and expensive instruments.
For example, it is now necessary for a person employed by a gas company to physically go to a gas source location or custody transfer station, take a sample of the gas being distributed and then return to the laboratory to determine its heating value. In the laboratory, two methods are most frequently used to obtain an accurate measure of heating value: combustion calorimetry, and chromatography along with a thermal conductivity detector.
Combustion calorimetry involves the burning of a partial stream of the combustible gas with an open flame or with a catalyst and measuring the heat produced. This procedure requires frequent maintenance of the apparatus since a flame can change due to deposits of combustion residues or because a combustion catalyst gradually declines in effectiveness.
Alternatively, a chromatograph along with a thermal conductivity detector is sometimes employed for determining the heating value of a fuel gas. After the sample is taken, a known amount of gas is introduced into the inlet of the chromatographic column. Typically, helium is used as the carrier gas, and as the column elutes each component of the gas, the thermal conductivity detector measures the changes in the thermal conductivity of the gas stream. The quantity of each of the constituents of the fuel gas is determined from these changes in thermal conductivity by a comparison to the changes produced by known quantities of the constituents. Since the heating value of each constituent is known, the total heating value of the gaseous fuel may be computed.
Both of these methods must be applied in a well-defined, controlled environment. The need for such an environment, along with the need to collect and transport samples and for complex measurement apparatus, obviously increases the cost of determining the heating value of the gas significantly.
To summarize, current methods used to determine heating value of a fuel gas are very costly, laborious and complex. Thus, a need exists for a process and apparatus for accurately measuring the heating value of a gaseous fuel such as natural gas, which is relatively simple, low cost and, preferably, applicable to unattended field operations. The need for an economical heat value meter suitable for unattended field type installations is particularly great when a distribution system is supplied by two or more gas sources. When multiple gas sources are involved, it is necessary to take measurements at many key points in the gas distribution system in order to ensure the accuracy of customer billing. With presently available methods, this is economically impractical.